audio/virtual_oss_bluetooth: New port

src commit 9cab9fde5eda ("virtual_oss: Port to base") includes the
bluetooth backend, however the backend also has a HAVE_LIBAV ifdef,
which, if set, enables libav support, which depends on
multimedia/gstreamer1-libav. While we could build the non-libav parts
from base and the libav ones as a port, we instead build the whole
backend as a port to avoid unnecessary complexity. The reason for that
being that building from base would install the backend in
/usr/lib/virtual_oss/voss_bt.so, and the port would install _another_
voss_bt.so, but this time in /usr/local/lib/virtual_oss/voss_bt.so,
which would create conflicts.

The port simply builds the files from src, which is not ideal, as this
means that the src repo needs to be present in the system in order for
the port to build, but this way we can avoid version mismatches and
ifdef hell as the project progresses.

Note that this is an extension for the base system virtual_oss, and not
audio/virtual_oss.

Sponsored by: The FreeBSD Foundation
Reviewed by: arrowd
Differential Revision: https://reviews.freebsd.org/D52426